On Monday, just after 3 a.m., Kim died from a pre-existing medical condition, according to Deputy Wally Bracks of the L.A. County Sheriff's Department.

According to prosecutors, the 77-year-old was feuding with a female neighbor at his retirement home when he decided to try to kill her. Investigators believe he may have also been trying to kill himself, as police said a note recovered from the scene suggests.

Kim allegedly set off an explosive device inside his apartment at the 11-story facility — triggering an emergency response by fire officials just before 4 a.m.

About 3:49 a.m., firefighters responded to a report of a fire alarm going off at the building, authorities said. Ten minutes later, they managed to put out the fire.

But with the smell of gas in the air and reports of an explosion, the firefighters stayed and continued to assess the safety of the building.

Long Beach Fire Captain Dave Rosa is seen in a photo released June 25, 2018, by the Long Beach Fire Department.

Meanwhile, Kim sat inside a hallway near his apartment, prosecutors said. And when Long Beach Fire Captain Dave Rosa started to walk down that hallway, he allegedly started shooting at him.

The 45-year-old fire captain died from his injuries later that same day.

The shooting also left two other people injured, including another man who lived in the building and firefighter Ernesto Torres, 35, according to authorities.

Rosa was a 17-year veteran of the department who left behind a wife and two children.

"You go to these scenes and you never know what's on the other side of those doors," Long Beach Police Chief Robert Luna told reporters hours after the deadly shooting.

"These brave firefighters went through those doors and unfortunately they were met with gunfire," he said.

The fallen fire captain was described as a funny, hardworking and loyal guy who loved baseball during a memorial service a little over a week later.

At the time, his 25-year-old son spoke of his shock upon learning of his father's death, as he remembered telling himself: "Firemen aren’t supposed to get shot."

"I cried that day for about 10 hours," Alec, his son, said.

"When I did stop, it wouldn’t be because I was in pain, or I ran out of tears," he said. "It was because my body was numb and it physically hurt to continue to cry."

The criminal charges that were facing Kim at the time of his death include one count of murder, two counts of attempted murder, one count of attempted murder of a firefighter and one count each of arson of an inhabited structure and explosion with intent to murder.

If he would have been convicted, he would have faced the death penalty or life in state prison without the possibility of parole.